Inkjet recording is a recording system for forming characters or images, comprising directly jetting ink droplets from very fine nozzles to a recording medium, and depositing the ink droplets on the recording medium. This system has some advantages such that not only the device shows excellent operability at a low level of noise, but also the coloration is facilitated and plain paper can be used as a recording medium.
With the widespread of the inkjet recording process, improvement in color image quality for specialized paper has been made. When the ink penetrates a paper, improvements in inks and paper have been made for contrivance not to cause bleeding, contrivance to increase drying speed, and contrivance to allow a coloring material to stay on a surface of paper, so that a high-quality printing can be accomplished.
On the other hand, improvements in color image quality against plain paper have been studied. For instance, a process for printing with two solutions containing an ink containing a colorant and a solution containing a polyallylamine has been known as disclosed, for instance, in Japanese Patent Laid-Open Nos. Hei 07-266689 and Hei 08-267903.
Also, some proposals have been made to improve color image qualities by using an ink set comprising a black ink containing an anionic dye and a yellow ink containing a cationic dye and a polyvalent precipitating agent as disclosed, for instance, in Japanese Patent Laid-Open Nos. Hei 06-57192; an ink set comprising a black ink containing an anionic pigment and a colored ink containing a component for supplying a cation as disclosed, for instance, in Japanese Patent Laid-Open Nos. Hei 11-123741; an ink set which induces a water-phase separation when an aqueous medium containing a first ink is contacted with an aqueous medium containing a second ink as disclosed, for instance, in Japanese Patent Laid-Open Nos. Hei 10-36724; and an ink set containing at least two colored inks which are gelated by lowering its pH as disclosed, for instance, in Japanese Patent Laid-Open Nos. 2002-363459.
However, these ink sets have a main purpose of suppressing bleeding. Therefore, there are some defects in that the ink sets have insufficient light-fastness when a dye is used as a colorant, and deficient optical density when a pigment is used as a colorant. Especially, when high-speed printing is carried out by lowering resolution, there is a defect that optical density becomes unsatisfactory.
There have been tried to increase the amount of the colorant contained in an ink or the amount of ink jetted, or change the composition of an ink, and the like for the purpose of increasing optical density of colored inks on plain paper. However, when high-speed printing is carried out at a low resolution, printed image still has unsatisfactory optical density on plain paper. Especially, when high-speed printing is carried out with a secondary color, the amount of the ink jetted becomes large, so that the penetration of the ink is accelerated, whereby a wide color reproducible range cannot be obtained.
In a black ink for which a secondary color does not have to be taken into consideration as in the colored ink, an ink set having two kinds of inks having different polarities from each other as black inks has been proposed as disclosed in, for instance, Japanese Patent Laid-Open No. 2000-7964. However, currently, optical density on plain paper cannot be said to be satisfactory even when two kinds of black inks having different polarities from each other are used.